Countershaft sprocket fell off yesterday on my 640.
On a ride out to a desert play area yesterday going about 75mph I pulled off an exit. Right as I rolled off the gas, CLUNK BANG, skidding rear tire for a second, then just coasting. I pulled to the side of the road and there was oil everywhere. I figured fuck, the engine case is broken. I might as well push this piece of shit into the great salt lake and be done with it.
Anyway I still am up in the air about what went wrong. The bolt was broken off inside the shaft. So it didn't just fall out. Or did it loosen enough to let the sprocket off the shaft then break off. I do check my sprocket from time to time and I have marks on the bolt/ washer and the sprocket to check it by just looking too.
Could the bolt have just failed from repeated overtorquing over the life of the bike? Or could it have just backed out even though it was loctited and checked once in a while? The bolt piece was still tight from loctite when I removed it with an extractor. And my reverse drills didn't make it move even though it grabbed. The piece left in the shaft was a little over 1/2 inch long.
I got the bike home and started looking it over. It was leaking so bad because the shaft spacer and oring came out. They were laying in my skidplate. I put them in and held them in place with my fingers. NO leaks.
The case looks ok. The main damage was the chain jamed in the frame and made a few little cuts and the lower guide got bent up. Luckily the chain popped off the rear sprocket and let everything freewheel instead of getting completely jamed in the frame and locked solid. The only question now is if the transmission is OK. It feels ok when I spin it in neutral. But who knows until I ride it again.
So the moral of the story is this. Don't just check those damn bolts, replace them once a season. I am ordering several sets of them now to just have them in the garage. They are really cheap....cheaper than a new engine.